Cryogenic power conversion for fuel cell systems especially for vehicles

ABSTRACT

A cryogenic power conditioning system for fuel cells is proposed which is cooled by liquid hydrogen or liquid natural gas (methane) feeding these fuel cells or by liquid nitrogen supplied by high-temperature superconducting cables. The main application are vehicles such as transit buses, etc. The result is higher efficiency, lower weight, smaller size and lower cost.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] A provisional patent application has been filed Mar. 15, 2000 entitled “High-Efficiency Power Conversion for Fuel Cell Systems, especially for Vehicles”. PPA Application No. 60/189,406, Mar. 15, 2000.

Statement Regarding Federally Sponsored Research and Development

[0002] None

REFERENCES TO A “MICROFICHE APPENDIX”

[0003] None

BACKGROUND OF THE INVENTION 1. Technical Field of the Invention

[0004] This invention relates to high-efficiency (>99.5%) power conditioning electronics used in order to convert the DC output of fuel cells into suitable AC power, especially useful for applications in vehicles, such as buses, trucks, ships, etc.

[0005] 2. Description of Related Art

[0006] Prior art is given by the existing technologies for transportation based on combustion engines. A worldwide effort is under way to implement new environmentally cleaner means of transportation by converting to electric propulsion. Fuel cells are considered for this application. F. Brief Description of the Invention According to news reports [1] General Motors developed a car called “Zafira” which operates on fuel cells supplied with the necessary fuel by liquid hydrogen. Liquid H2 exhibits a cryogenic temperature of 20.27 K (−252.88° C.). Other fuel cells use a reformer which extracts hydrogen gas from natural gas (methane, CH4) or gasoline [3, 4]. Natural gas may be available in liquid form at a temperature of −161.5° C. (112 K): LNG. Interesting possibilities exist in this case which will be part of this invention disclosure. Fuel cells deliver DC power which is then converted by socalled inverters into AC power required for general use and for the efficient operation of electrical motors. These inverters are in many cases larger in weight and size than the motors they are controlling. In addition they are also more expensive than the motors themselves. The new concept of Cryogenic Energy/Power Conversion (CEC, CPC) applied for the realization of a drastic size, weight and cost reduction in power electronics (Cryo-Micro-Power, CMP) as used in motor drives for transportation (or other) systems has a key problem: The cooling. But in the case that a cryogenic fuel fluid is already used such as liquid hydrogen or liquid natural gas, that problem is solved and interesting possibilities open up.

[0007] On the other hand, CEC can achieve its ultimate full potential only if implemented in the form of Multichip-Modules (CMCM) made with the wireless High-Density Innerconnect (HDI) technology [P6,11,12,13]. (Note: Size and weight reduction is nowhere more important than in vehicles if energy savings are important) It interconnects power transistor/diode chips in a half-bridge or full-bridge topology with polymer and metallic thin-films, thus eliminating the weak link in power electronics: The wire bond connections. Wireless interconnection provides many advantages such as higher switching speeds, higher frequency, better efficiency, improved reliability, etc.

[0008] The great potential of silicon applications in power and energy conversion (solid-state transformers, inverters, etc.) has not yet been adequately addressed by the semiconductor industry. It is desirable to change this situation with the promotion of the concept of cryogenic energy conversion discussed below in more detail. CMCMs are most needed in more efficient transportation systems. An application example is described in the following pages. Optimum systems can be designed if CMCMs are combined with High-Temperature Superconductors. The Multi-Chip Module will be the key component for the realization of the Cryogenic Energy Conversion concept finally bringing miniaturization to the field of high-power electronics (Micro-Cryo-Power), especially if combined with small filter inductors implemented with high-temperature superconductors.

[0009] The widespread application of electric vehicles requires the development of a sufficiently small, light, and efficient motor drive or ASD (Adjustable Speed Drive) between the fuell cell or battery output and the motor. Therefore multichip modules are proposed for an efficient motor drive system based on the following assumptions and suggestions:

[0010] Sooner or later, High-Temperature Superconductors (HTS) will be commercially available for applications in the power and energy generation and distribution fields at competitive prices: HTS cables, HTS transformers, HTS motors, HTS generators, etc. Billions of dollars have already been invested in this new HTS technology since its discovery in 1986.

[0011] HTS components require Cryogenic Cooling. In most cases, such as HTS cables and transformers, liquid nitrogen (LN2), at a temperature of <77 K (−196 C), will be used.

[0012] The availability of HTS components requires also a rethinking and redesigning of many energy systems. HTS Technology can best be supported by the new concept of Cryogenic Energy Conversion (CEC) based on Low Temperature Electronics (LTE) and Cryo-MOSFETs/Cryo-IGBTs, etc.

[0013] CEC can provide a considerable improvement in power and energy conversion efficiency as well as a drastic size, weight and, therefore, cost reduction: Micro Cryo-Power. CEC represents the mating of High-Temperature Superconductors with Low-Temperature-operated Semiconductors. Electronic efficiencies, without cooling penalty, of >99.5% should be possible.

[0014] The efficiency of electrical motor operation can be drastically enhanced by applying CEC to Motor Drives or ASDs (Adjustable Speed Drives). CEC would miniaturize these drives, which can be in prior art technology 2-3 times more expensive and much larger and heavier than the motors they control.

[0015] Size and weight reduction, along with improved conversion efficiency, is nowhere more important than in transportation vehicles. Every kilogram of weight reduction translates into a considerable energy saving for vehicles traveling hundreds of thousands of miles in a lifetime.

[0016] The push for higher efficiency leads to a push for electric vehicles requiring motors and ASDs.

[0017] Great progress has been made recently (New York Times, Oct. 21, 1997) in the field of Fuel Cells using gasoline or Liquid Natural Gas (LNG: 112 K, −161 C).

[0018] Therefore, this invention describes an ultra-small and light-weight Cryogenic Adjustable Speed Motor Drive in the power range of 50 to 200 Hp (35 to 150 kVA) using Cryo-MOSFETs or other suitable devices such as IGBTs.

[0019] Tremendous commercialization opportunities providing higher energy conversion efficiencies can be envisioned for many transportation systems combining (H₂, O₂) fuel cells using LNG, LH2, HTS motors, HTS cables and Cryo-Motor-Drives.

[0020] In the case that HTS motors are used, one can due to its small size integrate the cryo-power electronics inside the HTS motor or on the case of the HTS motor.

[0021] Motor drives using Cryo-Multichip Modules (CMCM) are intended for application in vehicles (buses, trucks, trains, ships, airplanes) as one important component in the coming age of Cryogenics which will combine High-Temperature SUPER-Conductors with Low-Temperature SEMI-Conductors. Such Adjustable Speed Drives (ASDs) will, of course, find other applications in stationary systems. Many manufacturing plants requiring ASDs already use liquid nitrogen for other purposes. The proposed CMD will provide smaller size, reduced weight and increased efficiency due to its application of the new concept of Cryogenic Energy Conversion (CEC). Every kilogram of weight reduction translates into a considerable fuel saving over the lifetime of a vehicle running hundreds of thousands of miles. Such a development should be desirable in view of the fact that the federal government now mandates that cities of certain sizes must provide alternatively-fueled methods of public transportation (“Cold Facts”, Summer 96 Issue). Nothing beats semiconductor technology as far as reliability, and size, weight as well as, therefore, cost reductions are concerned. It is finally time to apply this technology to the field of (high) power conversion. This is made possible by the concept of Cryogenic Energy Conversion (CEC).

[0022] A press release of Aug. 1, 1997 reads as follows: “Governor Pataki Announces Bond Act Funding for Clean Buses”. And: “Governor George E. Pataki today announced the State will award $3 million for the purchase of 39 clean-fuel buses as part of the Clean Fuel Bus Program under the Clean Water/Clean Air Bond Act.” These hybrid buses use CNG (compressed natural gas) to fuel a Diesel engine. They are described in an article by King et al., in the IEEE Spectrum of July 1995 [6] which presents the prior art. His figures [6, p. 29] shows how small the motor is compared to the inverter motor drive.

[0023] The next step in the development of the electric transit bus could be the use of fuel cells to replace the diesel engine as a power source. Many companies work on the development of such fuel cells. Very interesting possibilities exist in the application of the new fuel cell technology. For large vehicles such as buses, LNG (liquid natural gas) is desirable, while making the task of applying fuel cell technology easier. CVI, Inc. has successfully designed and manufactured on-board LNG fuel systems for buses such as those used by the Houston, Tex., METRO transit coach system.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0024] The concept of Cryogenic Energy Conversion (CEC) is based on the fact that certain semiconductor devices, especially high-voltage power MOSFETs (metal-oxide semiconductor field-effect transistors) work much better when cooled to cryogenic temperatures [M1-M18]. For example, the on-resistance R(on), a major source of loss, is reduced by a factor 20 to 35 by immersing the devices into liquid nitrogen (T=400 K/77 K). FIG. 1 shows the drain current dependence of R(on) for a 1000 V, 33 A MOSFET APT 10026JN. Up to a current of 55 A, the on-state voltage or resistance is absolutely stable and constant at 77 K.

[0025] In FIG. 2 the measured temperature dependence of R(on) is plotted for a 1000 V, 20 A, 0.53 Ω MOSFET APT10053LNR for drain currents of 1 A, 10 A, and 20 A. Assuming a maximum junction temperature of 100° C. (375 K) for normal operation (300 K) one obtains an R_(ON) improvement factor of 35 from 375 K to 77K.

[0026] The physics behind CEC is the drastic increase at low temperatures of the majority carrier electron mobility in the drain-drift region of a high-voltage power MOSFET. MOSFETs are the fastest switching power devices available [M8]. For a 100 Hz silicon motor drive the PWM (pulse-width-modulated) switching frequency of a switch-mode inverter can be low (1-20 kHz) so that switching losses are also small or negligible if soft-switching techniques are applied. The maximum efficiency is determined by the ratio of on-state voltage to the voltage swing. The APT MOSFET APT 10050 LVR, rated 1000 V, 21 A, and 0.5 Ψ (at 300 K), has an on-resistance of 24.2 mΩ at 77 K, i.e. immersed in liquid nitrogen (LN2). For a supply of 650 V, a current of 10 A, and 2 MOSFETs in series (as is usual in bridge circuits), the on-state voltage to voltage swing ratio is: $L = {{2\frac{0.242\quad V}{650\quad V}} = 0.00075}$

[0027] This corresponds to an conduction loss efficiency of >99.9%. Assuming a cooling penalty of a factor 10 and negligible switching losses at theses low frequencies an overall ASD inverter efficiency of >99.0% should be possible. By paralleling more MOSFETs one can further reduce the losses to any desirable level: “Silicon is cheap”! In addition, one should take into account the “load shedding” property of liquid nitrogen. LN2 can be generated in off-peak hours. Another advantage of CEC is the fact that the thermal conductivity of silicon and MOSFET chip substrates also improves drastically when cryo-cooled [M7]. The basic idea of CEC is to reduce loss from heat directly at the source by cryo-cooling.

[0028] Cryo-Multi-Chip Modules (CMCMs)

[0029] A conventional motor drive can be large compared to the motor it is designed to operate. Clearly, any reduction in size and weight (and consequently in cost) would be especially advantageous for transportation applications. Multi-Chip Modules have proven to achieve this goal in other industries (aerospace). However, this advantage has not yet been applied to the power electronics industry because of the high power densities involved which are now reduced by cryo-cooling. By using Multi-Chip Modules optimized for use at cryogenic temperatures, however, the entire volume previously dedicated to cooling systems for the motor drive circuitry can be removed. The resulting reduction in size and weight can be drastic.

[0030]FIGS. 3, 4, and 5 show various embodiments of the Cryo-Multi-Chip Module using the GE-CRD High-Density Interconnect (HDI) technology [11-13].

[0031] The proposed cryo-power motor drive will be an important component in the ultimate high-efficiency vehicle (transit bus, ship, truck, etc.) combining the emerging new technologies of advanced thin-film high-density interconnect technology (HDI), fuel cells, HTS electric motors and low-temperature power electronics.

[0032] The Cryogenic Electric Fuel Cell Transit Bus: The “Cryo-Bus”

[0033] Assuming that a fuel cell as described in the References [2-4] will be available some day in the future, one can envision an electric transit bus as shown in FIG. 6. A liquid natural gas tank operating at a temperature of 112 K may or may not be surrounded by a liquid-nitrogen tank (77 K) which would act as protective shield should an accident occur. Thus the Cryo-ASD motor drive can be cooled either by LN2 or by conduction cooling via ‘cold pipes’ or ‘cold fingers’ from the LNG tank The LNG tank supplies the fuel cell which generates also heat to be used for space heating in winter time. During the summer, the LNG tank could contribute to space cooling. The electric motors may or may not be implemented using HTS wires or conductors for even smaller size and weighl This invention limits itself to the miniaturization and efficiency optimization of the 4 motor drives through the application of the concept of CEC using CMCMs or cryo-MOSFETs. The liquid nitrogen tank may not be necessary and the cryo-power electronics may be located in a hermetically sealed case which is immersed in the cryogenic liquid natural gas. In this case the samll losses of the cryo-power electronics would help to evaporate the LNG for use in the reformer of the fuell cell. All cryogenic dewars, tubes, etc. are thermally insulated by multilayer insulation in a high vaccuum.

[0034] The concept of FIG. 6 applies, of course, also to a bus or vehicle where instead of LNG liquid hydrogen (LH2) is used. In this case the cryo-power electronics would be cooled in a space between the LH2 tank and the ambient at a suitable temperature gradient point in the range 77 K to 200 K. FIG. 7 shows the details of the cryo-bus enlarged.

[0035] A circuit topology as shown in FIG. 8 may be used for the implementation of a cryo-inverter used in combination with the fuel cell. The inverter circuit may be preceded by a boost converter to increase the low output voltage of the fuel cells. In the case where liquid nitrogen is used one can also use liquid oxygen (LO2) produced together with the LN2 for more efficient operation of the fuel cell. In this case it may be possible to eliminate the (necessary) pressurizing of the oxygen supplied to the fuel cell.

DETAILED DESCRIPTION OF THE INVENTION

[0036] The key objective of this invention is to apply the concept of cryogenic power conversion to fuel cell operated electric vehicles and other systems which use either liquid hydrogen (LH2) {GM car Zafira [1]} or liquid natural gas (LNG) in order to achieve the ultimate in high power conversion efficiency for environmentally friendly transportation. Here use is made of the fact that the cryogenics is already available and can therefore solve the cooling problem of the cryogenic power electronics plant 8. The advantages would be higher efficiency, lower weight, smaller size and lower cost for the required power electronics in a fuel cell operated vehicle.

[0037] The invention is demonstrated in FIG. 7:

[0038] A large vehicle such as a bus 1 uses a fuel cell (FC) 2 which converts oxygen and hydrogen into electrical energy. The wheels 3 are driven by the combination of an adjustable speed motor drive 5 and an electrical motor 6. The hydrogen needed by the fuel cell 2 can be provided by a liquid hydrogen tank or can be obtained via a reformer from liquid natural gas (LNG, CH4) stored in a LNG tank 8. The liquid natural gas tank 8 could, of course, also be a liquid hydrogen tank. The latter can be placed inside a larger tank 7 containing liquid nitrogen for protectiion in the case of an accident The motors 6 could use high-temperature superconducting wires for their windings and also be cooled by liquid hydrogen ot by liquid nitrogen. The cryomotor drives 5 could be integrated into and with the HTS motors 6 if multi-chip modules are employed. The cryo-power electronics 5 could also be placed in a total hermetically sealed case which is immersed in the LH2 or LNG tank 8 (not shown). The heat generated by the fuel cell 2 can be used for space heating 10 in the bus. The cryo-dewar 8 can also be used for space cooling 9 in summer time. The cryo-motor drives 5 are fed by electrical conductors 12 with a sutiable DC voltage. Cables 11 in turn feed electrical power to the motors 6. These conductors 11 could be high-temperature superconducting (HTS) cables which provide HTS bus motors 6 with electrical power as well as the cooling fluid (LN2). Tube equipment 13 delivers the natural gas or the hydrogen gas to the fuel cell 2.

[0039] The adjustable speed motor drives 5 can be implemented with conventional half- or full-bridge circuits, or an interesting new topology 20, the Stanley topology [9] shown in FIG. 8 can be used. The whole circuitry is placed inside a dewar exhibiting good thermal insulation walls, preferably using multi-layer vacuum insulation (MLI).

[0040] The motor drive circuitry if implemented in the topology of FIG. 8 consists of transistors, preferably MOSFETs, 21-24, controlled by drive circuits 25-28 and operated with pulse-width modulation (PWM). Commutating diodes 29-32 may be implemented with cryo-silicon or cryo-germanium devices. The latter have a much lower on-state voltage. Inductors 34-38 could be implemented with high-temperature superconductors. Together with capacitors 39 and 40 they form a filter for the elimination of the pulse-width modulation frequency. Inductor 36 constitutes the windings of the motors. The MOSFETs 21-24 and diodes 29-32 can be implemented in the form of cryo-multi-chip modules providing miniaturization. This motor drive circuitry can have an electronic efficiency (without cooling penalty) of greater than 99.7%.

[0041] It has been found that some gallium-arsenide based light-emitting diodes (LEDs) can exhibit 1-2 orders of magnitudes higher light output if cryo-cooled. Thus, cryo-cooled LEDs can be used in the Cryo-Bus of this invention for indoor and outside lighting applications. 

What is claimed is:
 1. A cryogenic electrical power conditioning system, especially for vehicles, comprising: a tank with a cryogenic fluid, an electrical power generation system supplying power to electronic motor drive circuitry cooled via said cryogenic fluid, said drive circuitry controlling at least one electrical motor, cooled by said cryogenic fluid.
 2. An cryogenic electrical power conditioning system according to claim 1 wherein the electrical power generation sytem is an oxygen/hydrogen fuel cell, which also provides space heating.
 3. A cryogenic electrical power conditioning system according to claim 1 wherein the tank contains liquid hydrogen, delivered to the fuel cell via tubes, said tank may be contained for safety reasons inside a second enclosure containing liquid nitrogen.
 4. A cryogenic electrical power conditioning system according to claim 1 wherein the tank contains liquid natural gas, delivered to the fuel cell via tubes and a reformer, said tank may be contained for safety reasons inside a second enclosure containing liquid nitrogen.
 5. An cryogenic electrical power conditioning system according to claim 1 wherein the motors, at least one in number, but preferably two to four, use coils and windings made by high-temperature superconducting materials, said motors are cryo-cooled via cold pipes connected to the cryo-tanks.
 6. A cryogenic electrical power conditioning system according to claim 1 wherein electronic motor drive circuitry using semiconductor switches, capacitors, and preferably high-temperature superconducting inductor coils, all cryo-cooled via cold pipes for improved efficiency is connected to said fuel cell power supply and said superconducting motors preferably via superconducting cables, said semiconductor switches to be selected from the set of cryo-MOSFETs, cryo-IGBTs, cryo-IGCTs, cryo-MTOs,
 7. A cryogenic electrical power conditioning system according to claim 1 wherein said power electronic circuitry is cooled by being placed into the temperature gradient environment between 20 K and 200 K for cryogenic cooling if LH₂ is used.
 8. A cryogenic power conditioning system according to claim 1 wherein said motor drive circuitry preferably using cryo-MOSE's is housed in a hermetically sealed container immersed for cryo-cooling inside said cryo-cooled environment of a liquid natural gas tank.
 9. A cryogenic power conditioning system according to claim 1 wherein said motor drive circuitry is conduction cooled by cold fingers connected to said liquid hydrogen/liquid natural gas tanks.
 10. A cryogenic power conditioning system according to claim 1 wherein even in the case of said fuel cells being powered by uncooled fuels, the whole power electronics, i.e. motor drives and high-temperature superconducting motors, is cryogenically cooled in order to achieve high efficiency, ultra-small size, ultra-low weight and low cost.
 11. A cryogenic power conditioning system according to claim 1 wherein high-temperature superconductor cables are used for power transmission as well as for supply of cooling fluid such as liquid nitrogen making use of the “load shedding” property of said liquid nitrogen.
 12. A cryogenic power conditioning system according to claim 1 wherein in the case of liquid nitrogen use, the fuel cell efficiency is increased by also using liquid oxygen produced together with the liquid nitrogen, eliminating the need to pressurize the oxygen used in the fuel cell.
 13. A cryogenic power conditioning system according to claim 1 which uses cryo-cooled semiconductor switches such as cryo-MOSFETs, cryo-IGBTs as active devices and cryo-diodes made of materials selected from the group of silicon and germanium for commutation in a circuit selected from the group of half-bridge, full-bridge, three-phase, and Stanley topologies, preferably using optical coupling to the gate control terminals, where the cryo-MOSFETs, cryo-IGBTs and cryo-diodes are implemented in the form of Cryo-Multi-Chip Modules providing small size and low weight which is especially important for all transportation systems, where the driver circuits are implemented by cryo-cooled integrated circuits, where the necessary filter inductors and power busses of the inverters are implemented with high-temperature superconductors.
 14. A cryogenic power conditioning system according to claim 1 where the cryo-fuels are also used to cool a cryo-light system based on high-efficiency cryo-light-emitting gallium-arsenide-based diodes. 